The invention generally provides a transfer switch for automatically connecting the neutrals of one or more loads to an alternate power source, such as an electric generator, when a transfer to alternate power is made from primary power, such as a utility, and for automatically reconnecting the neutrals to the primary power supply when a transfer back to primary power is made.
A transfer switch is an electrical switch that connects an electrical load to a standby or auxiliary power supply when electrical power from a primary power supply, such as a utility power supply, has been lost. Transfer switches can be classified into one of two groups: manual transfer switches and automatic transfer switches. Manual transfer switches require a user, such as a homeowner, to physically throw the switch to connect electrical loads to the standby power supply, which in many instances is an electrical generator. When the primary power supply has been restored, the user must then throw the switch back to its previous position to reconnect the electrical loads back to the primary power supply.
An automatic transfer switch, on the other hand, is designed to automatically connect the electrical load to the standby power supply upon the loss or interruption of primary electrical power. Automatic transfer switches are often installed where a backup electrical generator is located so that the electrical generator can provide temporary electrical power if the primary power supply fails. In this regard, as well as transferring the electrical loads to the electrical generator, an automatic transfer switch also commands the electrical generator to start based on a disruption in the primary power supply. The transfer switch also isolates the electrical generator from the primary power supply when the generator is on and is providing temporary power.
Thus, for example, in a home equipped with a backup generator and an automatic transfer switch, when an electric utility outage occurs, the automatic transfer switch will signal the backup generator to start. Once the automatic transfer switch sees that the generator is ready to provide electric power, the automatic transfer switch breaks the connection to the electric utility and connects the generator to certain selected electrical circuits in the building, which are typically those circuits that power critical items or equipment such as a sump pump, furnace, refrigerator, medical equipment, etc. If the generator is large enough, it may be connected to all of the loads in the building. The generator supplies electrical power to the electric loads, but is not connected to the electric utility. Isolation of the generator from the electrical distribution system is required to protect the generator from overload, and to prevent accidental energization of the service wiring. When utility power returns for a set time, the automatic transfer switch will transfer back to utility power and command the electrical generator to turn off, after another specified amount of “cool down” time with no load on the generator.
In addition to being classified as manual or automatic, transfer switches can also be classified as either separately derived systems or non-separately derived systems. In a separately derived system, there is no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors of the electrical generator. On the other hand, in a non-separately derived system, the electrical generator has a “solidly connected” grounded circuit conductor. In the case of the latter, the grounded conductor, i.e., neutral, of the electrical generator is solidly connected to the neutral conductor of the main electrical panel. In the case of the former, however, the transfer switch must switch the neutrals. That is, the transfer switch must switchably connect the neutral conductor of the electrical panel to the neutral of either the electrical generator or the utility power supply. So, for use in a separately derived system, the automatic transfer switch must also switch the neutrals automatically upon the loss of primary electrical power and the subsequent return of primary electrical power.
One proposed automatic switch for switching the neutrals uses a contactor that switches the neutral conductor of the electrical panel to the electrical generator neutral whenever a running generator is connected to the system. More particularly, the contactor has a coil that is energized when electrical current flows from one generator hot conductor to the generator neutral. Energization of the coil causes an armature to move the neutrals switch to connect the panel neutral conductor with the neutral conductor of the electrical generator. When the electrical generator is shut-off or otherwise not supplying electrical power to the load center, the coil will become de-energized and the armature will be released to allow the neutrals switch to reset and reconnect the neutral conductor of the load center with the neutral conductor of the primary power supply. This design, however, allows the transfer switch to reconnect the hot conductors of the electrical panel back with the hot conductors of the primary power supply when primary power is restored but, if the electrical generator is still running and connected to the transfer switch, the neutral of the electrical panel will not be switched back. This makes primary power, e.g., utility power, available to the loads of the electrical panel with no neutral connection. As a result, either a load on the electrical panel may receive electrical power at high, and potentially damaging, voltages or the ground wire may conduct current, creating a potential safety issue. The exposure to undesirable high voltages may also occur during post-installation testing when it is not uncommon for an installer to test operation of the automatic transfer switch and electrical generator with the primary supply on and connected to the electrical panel.
The present invention is directed to a method and system of selectively controlling the supply of electrical power to an electrical panel that is capable of receiving electrical power from more than one power source, such as a utility power supply and an auxiliary power supply, e.g., electrical generator. More particularly, the invention prevents the neutral conductor of the electrical panel from being connected to one power supply when the hot conductors of the electrical panel are connected to another power supply. In this regard, the present invention prevents the neutral conductor of the electrical panel from remaining electrically connected to the neutral conductor of the electrical generator when primary power has been restored and the hot conductors of the electrical panel have been reconnected to the hot conductors of the primary power supply. In a similar fashion, the invention prevents the neutrals from being switched to the electrical generator when primary power is feeding electrical power to the electrical panel.
The present invention uses the voltage supplied to the transfer switch, i.e., the transfer switch bus, to determine the appropriate position for a neutrals transfer switch. When the voltage potential or drop between the transfer switch bus and the primary power supply is at or near the rated voltage of the power supply, e.g., 240V, the neutrals transfer switch connects the neutral conductor of the electrical panel to the neutral conductor of the primary power supply. On the other hand, when the voltage potential between the transfer switch bus and the auxiliary power supply, e.g., electrical generator, is at or near the rated voltage of the auxiliary power supply, e.g., 240V, the neutrals transfer switch connects the neutral conductor of the electrical panel to the neutral conductor of the auxiliary power supply. In this regard, the switching of the neutral conductor of the electrical panel is based on which power supply is feeding the transfer switch bus.
In one embodiment of the invention, a contactor having a coil and movable armature is used to detect the voltage potential between the transfer switch bus and the primary power supply or auxiliary power supply. In another embodiment, a microcontroller and associated sensors may be used to measure the aforementioned voltage drop and control relays or switches to make the neutrals connections.
One object of the invention is to provide a stand-alone switching apparatus for use with a transfer switch that automatically and independently switches the neutrals of a separately derived transfer system.
It is another object of the invention to provide a neutrals switch for use with a transfer switch in which the neutrals switch is integrally formed with the transfer switch yet independently connects the neutral of an electrical panel to one of a pair of power supplies based on which power supply is energized to provide electrical power to the transfer switch.
It is a further object of the invention to provide a neutral switching apparatus that prevents the neutrals of a transfer switch system from being electrically connected to a power supply that is different from the power supply that is feeding electrical power to the transfer switch system.
Various other features, objects and advantages of the invention will be made apparent from the following description taken together with the drawings.